What Is Baylisascaris procyonis?

Ron Hines DVM PhD

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Some viewers, came to this page because they are caring for an orphaned wild raccoon. But my advice might be just as helpful if you have a nuisance raccoon problem, keep one as a family pet, or if you just want to understand the risk that Baylisascaris-exposure might cause to you and your children. These are some things you can do to minimize that risk.

About Baylisascaris:

Intestinal roundworms are nematode parasites. Almost every species of animal (host) – including humans – have their own special types of nematodes that have adapted to thrive in their intestinal environment and that have developed successful ways of transferring from one host to another.

Raccoons are no different. Their intestinal roundworm is Baylisascaris procyonis. Over the eons that they have existed together, the raccoons and this parasite have worked out their relationship well enough that the raccoons normally suffer no health issues when the parasite is present in their intestines in reasonable numbers. The threat to people occurs because of the special way this parasite has adapted to move from one raccoon to another and the cleverness of raccoons in adapting to our urban landscape (high ecological plasticity). (read here) Raccoons are exceptionally clever, adaptable omnivores, equipped with thumbs which allow them to, among other things, open garbage containers and food receptacles. Although raccoons are native to North America, you might not know that they are now common in Central Europe and Japan and parts of the former USSR. (read here) Unfortunately the raccoons have brought the baylisascaris parasites along with them. You can read more about that here.

Who Is Most At Risk? What Are The Actual Chances Of Me Or My Family Becoming Infected With This Parasite?

Anyone who inhabits an area where raccoons live is potentially at risk. Young children and mentally people are at higher risk of infection because they are more likely to put contaminated fingers, soil, water or objects into their mouth. That is the only way we know of to become infected. You can read accounts of various human cases here. This same feces-to-oral contamination rout is how dog roundworms (Toxocara canis) infect humans. Anywhere between 1-10% of American children show evidence in their blood of having ingested dog toxocara eggs. The more likely children are to play in dirt, the more likely the exposure. 

Scientists know much less about the frequency of humans exposure to Baylisascaris. In a 2003 study of children in the Chicago area, 30 of 389 (8%) showed evidence of exposure to Baylisascaris in their blood. There were problems with the blood tests used in that study. The authors told me that exposure of these children to dog roundworm eggs might have been inadvertently attributed to exposure to raccoon roundworm eggs. The CDC has stated that “predictive value of available serological tests [available at that time] are unknown”. 

Fewer than 20 serious human brain infections (encephalitis) from raccoon roundworm have been documented in the last 30 years. Over the same period, there have been an average of 55 lightning fatalities in the US each year and 75 cases of amoebic encephalitis due to swimming in dirty water. Of course, if you or your family handle or expose yourself to raccoon waste in unsanitary ways – all bets are off.

Is The Risk Of Infection Becoming Greater?


That is because the urban raccoon population is quickly increasing. Public health departments harp on the dangers of baylisascaris in an attempt to deal with rising populations of marauding urban raccoons, and they are correct in doing so. But the dangers of these large populations of clever, unsupervised animals is really much broader than that one threat. Abnormally high raccoon populations disrupt Nature’s harmony and balance. Much like feral cat colonies they destroy the smaller wildlife and bird population and bring the threat of raccoon-borne rabies as well as raccoon-borne Lyme disease and raccoon-transmitted leptospirosis into your neighborhood environment.

Feral (stray) cat colonies and urban raccoon problems go hand-in-hand. It is not only the cats that come at night to feed at the feeders that well-meaning people supply for those cats.

There is a second, potentially greater, risk. Baylisascaris can also live in the intestines of dogs. As dogs and urban raccoons mingle, there is a real possibility the more dogs will become infected. You can read about that threat here and here. Dogs relieve themselves wherever convenient – not in specific latrines. In doing so, dogs contaminate the environment much more widely and make human exposure to their parasite eggs considerably more likely.

What Is The Natural Life Cycle Of This Parasite?

Baylisascaris is a species of roundworm or ascarid. It lives its whole adult life in the small intestine of raccoons. All ascarid parasites do little damage in their adult form. They absorb the nutrients they need from the partially digested food in the raccoon’s intestine – not by damaging the raccoons’ intestine itself. So unless the raccoon has an usually large number of these parasites, they cause it little or no harm.

Baylisascaris exists as either a male or a female worm. When female worms mature (in about a month or two), they begin to lay an enormous number of eggs that leave the raccoon mixed with its stool. Raccoons under a year of age are usually the largest shedders of these eggs. When raccoon stool is fresh, the eggs are not infective. But after the stool has sat for 11-14 days, the Baylisascaris eggs have developed sufficiently to hatch if eaten. In this infective state, in the proper conditions, these eggs can remain infective for years. Two things can happen to these infective eggs. If they are accidentally consumed by another raccoon – particularly an immature raccoon – they can develop into new mature parasites in that animal’s intestine.

A second, probably more common thing can also occur. The infective eggs can be consumed by another species of animal – usually a small rodent or perhaps a bird. In those non-raccoon species, the eggs do hatch. But instead of staying in the new host’s intestines, the larval parasites migrate throughout the host’s body. They are eventually walled off in small islands (cysts) of inflamed tissue (eosinophilic granulomas). Perhaps more frequently than with other ascarids that share this type of life cycle, Baylisascaris granulomas occur in nerve tissue or the brain. These debilitated rodents and birds are easy prey for raccoons. When they are eaten, the encysted parasites in these small prey animals find their way back to the raccoon’s intestines where the cycle begins anew.

Occasionally, this parasite will enter the eye of these non-raccoon species causing severe damage. (read here) Baby raccoons as young as three months old can already be passing baylisascaris eggs in their stool. (read here)

Is Baylisascaris More Common In Raccoons In Some Localities Than Others?

Probably so.

Studies of raccoons in various parts of the US have found different rates of infection. Baylisascaris eggs do not survive well in the dry, hot conditions found in some areas. Urban raccoons are more likely to consume garbage, and dog and cat chows than their normal infected prey animals so perhaps, they have slightly less exposure to Baylisascaris through the prey they would normally eat. Yet diseases of all kinds tend to spread more quickly in crowded condition. We do not know how those two factors might interplay to influence parasite numbers. Because immature and young-of-the-year raccoons tend to have more parasites, Baylisascaris seems to be more common in late summer and fall when that year’s raccoon offspring are maturing and prevalent. 

What Happens When These Parasites Get Into A Human Being?

Should you, or a family member inadvertently swallow an infective Baylisascaris egg, it will continue with its development just like it would have in the rodent or bird intermediate host that it was designed to infect.  It will hatch in your digestive system, the larva will penetrate through the intestinal wall and begin its wandering throughout the body. All intermediate hosts recognize this larva as a foreign invader (foreign protein) and attempt to wall it off with defensive cells and tissue (granuloma). If this granuloma forms in a non-critical area, it is not of much importance. With extended time, your body defenses will kill and absorb it. If, however, this granuloma forms in the brain, eyes, or spinal cord it will cause severe damage. Often, that damage, once apparent, is permanent. Occasionally, it is not. This occurs with no apparent symptoms in the victim and with no one the wiser that such a thing occurred. If, however, the Baylisascaris larva lodges in the victim’s brain or eye, the body’s attempt to destroy it can damage that organ quite a bit. In those cases, loss of vision, paralysis and mental changes all occur. Even dog roundworms occasionally cause this. (read here)

One of the body’s defense cells, responsible for fighting foreign invaders like Baylisascaris, is the eosinophil. A high eosinophil blood count (eosinophilia) is often the most reliable clue to physicians that a parasite like Baylisascaris might be responsible. Should large numbers of these raccoon roundworm eggs be eaten, the symptoms are likely to be much like those seen in children (or adults) who have consumed large numbers of dog roundworm eggs (Toxocara canis) by accident. (=visceral larval migrans). It has been difficult for physicians to diagnose Baylisascaris infections in humans with accuracy. However, a recently developed form of the ELISA test may be effective in doing so. You can read about that test as it pertains to baylisascaris detection here.

What Adds To A Person’s Risk?

The first and most important risk fact is the number of raccoons living in your immediate area. In some suburbs and cities in the US, raccoon populations are enormously dense – up to 383 raccoons/square mile (238/km2). The problem is particularly severe in California and Florida. Young children are most at risk of consuming parasite eggs of all kinds – including Baylisascaris. This is because of their tendency to put things in their mouth, their curiosity, and their inability to stand. Young of all species also lack developed ways of fighting these parasite larva once they enter the body. People who practice poor hygiene for one reason or another are at greater risk. These can be folks with diminished mental capacities or those whose work demands that they enter contaminated areas frequently, or who increase their exposure to raccoon feces for one reason or another. Similar occupational hazards exist when it comes to dog roundworms. You can read more about that issue here.

What Specific Steps Can I Take To Prevent Exposing Myself Or My Loved Ones To This Parasite? How Can I Get Raccoons To Leave And Live Somewhere Else?

Spread information about the dangers of feeding raccoons to people you know. This is basically a people problem, not a raccoon problem. Raccoons are one of the cutest of wild animals. One look at a baby raccoon or momma raccoon and her brood begging for food at your door is enough to melt the coldest of hearts. It is just natural that we would want to feed and shelter them. It is not apparent to most people that doing so upsets the natural balance of Nature and exposes our families to disease. There is little that one person or one family can do on their own to decrease raccoon numbers. It takes the combined work of an entire community.

Do not intentionally feed raccoons or leave pet food outside unattended. Dry dog and cat chow, left outside for your pets, is the biggest single factor allowing urban raccoon populations to explode. Dogs and cats do perfectly well on one or two feedings a day. There is no need to maintain outside feeders. Feeding stray cats and dogs, or having neighbors who do, just perpetuates the pet over-population problem. Maintaining feral cat populations is very short-sighted. It has just as negative an effect on long-term pet and human health and well-being as feeding raccoons. Read more about that here. If you do feed your pets outside, taking the food in after 4PM is helpful. However, in many localities, raccoons are becoming daytime feeders as well as nighttime feeders. The old idea that raccoons, out in the daytime, have either distemper or rabies is no longer true.

Secure Your Trash Receptacles.

Raccoons are amazingly clever and persistent in getting to garbage and human food sources. They have grasping abilities that no other American wildlife possesses. They are extremely clever and patient when they know that a food source is nearby. Long ago, urban raccoons learned to associate the mere presence of humans will easy food. Clean up after your family picnics. Don’t rely on park trash receptacles – take the waste home with you instead. See to it that dumpster lids remain closed. Put your trash can in a wooden or metal frame that prevents it from being tipped over when raccoons come a calling.

Keep your property as free of nesting and hiding sites as you can.

Cover any entrances to crevices, nooks and crannies with chicken wire. Do the same for any entrances into your roof or attic. (When blocking a potential entrance, bend the chicken wire into a 90-degree angle that extends 12 inches on the ground ahead of the opening. Raccoons will easily dig under a barrier placed at ground level; but they will not begin digging 12 inches in front of the entrance.) Firewood piles are an ideal place for raccoons to live in and for them to establish latrines on. Consider keeping the firewood in a shed, or on an off-the-ground rack surrounded with chicken wire. Remove as much trash, brush and items that raccoons might hide in from your property as possible. But remember, no matter how inviting raccoons might find your property to live on, they will not live there unless they have nearby access to food.

Use flashing on objects raccoons might climb.

Raccoons gain access to attics and roof crevices by climbing trees. Cut and trim back branches that come anywhere close to the roof of your house. A 16-inch surround of roof flashing secured by a nail will prevent raccoons from scaling these trees. (here is a photo)

Take in or empty bird feeders at sundown. I have found chemical Repellents to be ineffective. I have not found any chemical animal repellents to will dissuade raccoons from using a food source when they are hungry. The same goes for ultrasonic contraptions and motion-activated devices – Raccoons soon get over their initial fear of these new gadgets and entirely ignore them.

What About Humane Trapping And Relocation?

Raccoons are rather easy to catch in live traps. Traps work well to eliminate a particular nuisance individual or small band of raccoons. However, there are really no suitable places to relocate these animals to and if your home and yard remain attractive to raccoons, new ones will soon appear.

What About Your Dog?

Some dogs accept the presence of raccoons on their property, some do not. Raccoons are quick to find out which. Keep your dog on a monthly heartworm preventative that controls Toxocara canis (dog roundworms). Studies have not been done, but those same medications are likely to kill baylisascaris as well.

Dogs can serve as either the final or the intermediate host of baylisascaris. That means that the raccoon parasite can both make your pet ill and live in its intestine as a source of infection to other family members. That should not be a problem if you keep your pets on a heartworm preventative all year long. Cats do not appear to be suitable hosts for the adult worm – but they are equally susceptible to disease caused by the immature baylisascaris larva.

How Long Do These Parasite Eggs Last In The Environment – How Tough Are They?

In relatively moist, shaded conditions, Baylisascaris eggs will last a very long time (years) – longer than you would wish to wait. Remember, Baylisascaris eggs in fresh raccoon feces are not the threat. They must develop, on the ground or in the soil, over a 2-4 week period before they can infect another animal (or person). Near-boiling water temperature will kill baylisascaris eggs, but drying, winter temperatures or bleach disinfection have little effect on them. They are likely to even survive a trip through your drain pipes and on to your local sewage plant. (read here) Other animal roundworm eggs are more easily killed by ammonia than by bleach. We do not know but perhaps Baylisascaris would be as well. 

How Can I Effectively Clean Up An Area Of Raccoon Feces And Destroy The Parasite’s Eggs?

The best method to deal with a contaminated area or raccoon latrine is to use a square-ended coal shovel to scrape up the feces and about the top 1-2 inch of soil. First lightly mist the area with a garden hose or spray bottle of ammonia solution to keep dust down. Place what you remove into a heavy-duty garden leaf bag and put that in your trash can. When cleaning an area, wear a face mask (N95-rated) and gloves. Keep children, pets and bystanders away. When you are done, put all the clothes you wore through a hot water washer and dryer cycle. Then take a shower, wash your hair, clean your fingernails. Do the job on a day that is not windy. Raccoons often defecate in elevated areas, decks, attics or the bases of trees that are hard to clean. You will need to wear nitrile gloves when handling these contaminated objects. Never use a leaf blower or anything that would scatter the eggs even more. An alternative is to cover the area with an additional 4-6 inches of new soil, lay down plastic sheeting or some other impervious material. Items that can be rinsed with soapy water should be thoroughly scrubbed, and the rinse water poured down the toilet. Throw away all items used in the process. Hard smooth surfaces can be disinfected with boiling water or a steam jenny (Some of these jennys reach 325 F/163 C at the nozzle – so be careful and test a small piece of carpet or sofa material to be sure you do not ruin them if you are steaming in or near the home or attic). A propane torch works equally well where there is no fire hazard. I once had a neighbor burn down his garage while attempting to flame a wasp nest – be careful! If you do not want to face these potential hazards, hire an experienced, bonded cleaning company to do it for you.

Are There Medications That Will Effectively Rid Raccoons Of Baylisascaris?

If you keep raccoons, raise orphan ones or if you insist on feeding wild ones around your home, mature baylisascaris are thought by many to be rather easy to kill. Just assume that all raccoons have them. I suppose that a baylisascaris-free raccoon over-population is better than a baylisascaris-carrying raccoon population. A study performed in 1994 found that a number of worming medications mixed with food baits were effective in ridding raccoons of baylisascaris (oral pyrantel pamoate,   ivermectin,   moxidectin,   albendazole,   fenbendazole and flubendazole). (ask me for Bauer1994) A second study also found that oral pyrantel pamoate was effective. (ask me for Page2011)

But if you keep raccoons, there may be some danger in assuming that a raccoon that is confirmed to not be shedding baylisascaris eggs in its stool is truly free of the parasite after it has been wormed. It might be that the medication has simply blocked the parasite’s ability to produce eggs at that time. Adult female dog heartworms, another nematode, sometimes behave that way. The most accurate determination that a raccoon is free of baylisascaris is probably to run a PCR test on its stool. (read here) This test is available commercially.

Are There Effective Treatments Once This Parasite’s Larva Enter The Brain Of Animals Or Humans?

Once Baylisascaris has entered the brain, eyes or some other place in the nervous system of a non-raccoon species, treatment is very difficult. That is because most of the damage that is occurring is not due to the parasite itself. It is due to the body’s reaction to the parasite’s presence. Even when that parasite is destroyed by medications, the offending presence of the dead parasite is still there. In fact, if the parasite disintegrates, the situation can be made worse. So, treatments usually focus on blocking the body’s immune response with medications like prednisone and providing supportive nursing care. In certain instances, such as a baylisascaris larva present in the eye, the parasite has been successfully removed surgically or destroyed with a medical laser. When attempts are made to kill the larva with medications, it is often albendazole that is used. (read here & here)

Aaron Writes:

I was reading this webpage and noticed that you mentioned how unusually resistant Baylisascaris procyonis eggs are to heat. A while ago I ran across an article in which researchers determined that the ‘100% thermal death point’ of baylisascaris procyonis eggs was just 62 degrees C (144 F) sustained for some unspecified time but less than one minute. (read here) I always thought that typical sterilization temperatures were generally well in excess of 100 C. If this research is correct, then submerging a potentially infected object such as feeding bowl or toy in boiling water for one minute or running it through the ‘heated dry cycle’ on most consumer dishwashers (usually thermostatically maintained at 75 or 83 C) would be sufficient to kill any eggs on the object. With regard to potentially infected bedding materials, I suspect that an ordinary clothes dryer could achieve sustained internal drum temperatures of 62 C or more once most of the water had evaporated from whatever was being dried.

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